1. Field of the Invention
The present invention relates to electrical power supply from a variable fuel cell power supply system for reliably generating ac and dc power as a stand alone unit.
2. Background Art
The availability, defined as percentage of run time over total time; the reliability, determined as the number of occurrences of failure per unit time of operation; and the durability, defined as service life, of previously known fuel cell generators are not sufficient for a free-standing, continuously operating, dynamic load-following, local fuel cell application using currently available manufactured components or even improved components.
Previous claims to fuel cell availability, reliability and durability, as reported in newspapers, magazines, and technical journals, are specific to a particular existing or envisioned application. For instance, the durability needs of a fuel cell automobile engine (e.g., 5,000-10,000 hrs) with its intermittent operation may not be acceptable in a residential application with continuous operation (e.g., 45,000-90,000 hrs) requirements. In addition, a fully acceptable 85-90% operating availability for a manned, grid backed, utility site would not be accepted by the general public for a local, unmanned, residential unit (where expectations are more likely to be one loss every 5-10 years). A NASA space fuel cell application using pure stored liquid hydrogen and oxygen cannot be relied upon by skilled artisans to establish how to improve reliability to required levels for a free-standing, re-formed fuel, ambient-air terrestrial application, particularly under dynamic and transient operation.
Improvement efforts focus on components rather than systems to solve availability, reliability, durability, and excessive cost problems. A non-exhaustive list of specific Proton Exchange Membrane (PEM) fuel cell problems currently under attack include: carbon monoxide/dioxide catalyst poisoning from reformed hydrocarbon fuels, membrane hydration control for dimensional stability and ionic conductivity, membrane physical integrity for pressure differential and expansion or contraction stress failures, membrane dehydration and electronic failures caused by hot spots from poor heat dissipation, and fuel cell contamination caused by incomplete fuel reforming and/or cleanup. Most fuel cell problems are accentuated by operational dynamics and/or transients. Conventional wisdom demands that systems must shutdown processes in the event of hydrogen leakage. In addition, manufacturing process variability for membranes, electrodes, and Membrane Electrode Assemblies (MEA's) result in a lack of consistency or repeatability in performance (or operation) for assembled fuel cell stacks.
In an attempt to solve these problems, previous research and development efforts have focused on components. New materials for membranes, bipolar plates, catalysts, composites, and electrodes have been developed. Geometry changes, including stacked plates, round, hexagonal, and others have been tried with varying success. Improvements by studying the chemistry of contamination, chemical process stability, chemical degradation, chemical depletion, and alternative chemistries, have been sought. New processes for manufacturing components have focused on cost reduction, tolerances, and uniformity. Improvements in operational safety and the effectiveness of system monitoring and control have been sought through the development of controllers and sensors that possess integrity during variations in pressure, temperature, and humidification. The need to use fossil fuels, because of the lack of a hydrogen distribution infrastructure, has caused development of storage, distribution, reformer, multi-fuel reformer, and contaminant clean-up equipment. Finally, improvements in ancillary fuel cell generator components such as energy storage units, electrical converters, compressors, pumps, and manifolds have been tried with varying success.
There are also conflicting issues that require different trade-offs for different applications. Previously known improvements, such as thinning membranes to increase conductivity and fuel cell power density, reduces reliability by compromising physical integrity. Membranes expand and contract as much as 20% between their dehydrated and fully hydrated states, creating opportunities for physical failure due to stress, strain, creep and tear. In addition, gas pressure differentials across the membrane are more likely to cause membrane punctures. When reducing membrane dimension, membrane thickness and uniformity becomes more difficult to control. Lightened catalyst loading decreases cost and reduces durability due to increased susceptibility to catalyst poisoning from fuel or oxidant impurities.
In addition to the fuel cell stack issues, the remaining plant devices also contribute to the operational, reliability, and durability problems. For example, inverter failures have significantly reduced the availability and durability of prototype fuel cell generators and commercial Uninterruptible Power Supply (UPS) installations. Both single fuel and multi-fuel reformers, using commonly available fuels, are still in the R&D stage for attaining hydrogen-rich and contaminate-free operation, particularly under transient conditions. Batteries have well known duty cycle and durability issues that have led researchers in the field toward storage alternatives such as ultra capacitors and fly wheels.
Virtually all major automotive manufacturers are targeting 2004 for the introduction of commercial Proton Exchange Membrane fuel cell engines of a size that outputs approximately 50 kW. Fuel cells become cost-competitive with the internal combustion engine at approximately $35-$50 per kW. A large scale phosphoric acid fuel cell (PAFC) stationary commercial supply system is produced by ONSI, the PC25; and Ballard Power Systems has demonstrated a 250 kW PEM prototype. The target market for these products are utilities, industries, hospitals, and commercial establishments because of their size. A small scale stationary/residential system has not yet been commercialized, although Plug Power has set a residential PEM fuel cell commercialization target of 2001 for a supply output size of approximately 7 kW, while APC, Avista, Energy Partners, H-Power, and NPS have all stated intentions to enter this market. Fuel cell generators become cost-competitive at approximately $500-$900 per kW in this market.
Fuel cell developers are looking for synergy between the transportation/automotive and stationary product markets. However, the previous developments do not establish that improvements and cost reductions will always be directly transferable between products/markets. Auto manufacturers, suppliers and their strategic partners are addressing problems through research and development at the component level and through the simplification of systems by component reduction, however, transportation application requirements allow tradeoffs that are detrimental to stationary applications.
Large stationary fuel cell system developers are accepting the low reliability specifications of existing utility generating equipment. The exception is a developer (SurePower) who is targeting high-value power quality with the application of ONSI Corporation's 200 kW PC25 units. However, the SurePower approach relies upon rotary equipment as well as grid connection and generators to compensate for fuel cell production inconsistencies during dynamic and transient load changes. Some small Stationary/Residential system developers including some fuel cell suppliers and their strategic partners appear to be addressing reliability and durability problems with a grid interface for alternate supply where the grid becomes an energy storage buffer and backup for the fuel cell.
Fuel cell technology is generally touted for reliability because there are no moving parts and availability exceeds that experienced for the most reliable utility generators. However, the current state-of-the-art fuel cell generator falls short of the needs for stationary free-standing and/or high power quality markets. The ancillary equipment needed to support the fuel cell in an environment with no hydrogen fuel distribution infrastructure, and a need to convert the dc output to ac for most applications, adds complexity and additional failure modes that reduce availability significantly. Most components are not market mature (reformers, sensors, controls, membranes, etc.) and their reliability is unproven. Where ancillary equipment is mature (inverters, batteries) the devices are not known for their reliability and durability and do not provide for secure, unattended operation.
Much of the current research and development effort is targeted at improved fuel cell materials, catalysts, and fuel cleanup all driven by reduced cost targets. However, many of the cost reduction efforts have potentially degrading effects on the fuel cell reliability, availability, and durability experienced to date. The use of thinner fuel cell membranes reduces cost and weight while sacrificing physical strength and integrity. Lower catalytic loadings reduce costs while increasing the risk of catalyst poisoning and perhaps reducing durability. New materials in bi-polar plates reduce costs but may degrade operating characteristics and conductivity. New materials and the processes producing them could introduce impurities that would detrimentally affect the long term stability of the fuel cell's chemical reactions. Even the strongest supporters of fuel cells acknowledge the performance risks associated with taking this technology to market at a competitive price.
In a commercial utility grid, the interconnected grid is divided into control areas and each area is the assigned responsibility of a specific utility. The control strategy uses the mechanical inertia of rotating electrical equipment, generator governors set at 60 cycles, and supplemental generator control (using unloaded spinning reserve), to maintain the area control error (ACE) and grid frequency within specified ranges under normal dynamic loading conditions. In simplified form, ACE equals the difference between the area's load and the sum of the area's generation plus purchase power contracts minus its sales contracts. A non-zero ACE indicates an unbalance in the load and supply. For supply deficiency, a signal derived from ACE is used to load spinning reserve, and for supply excess, the ACE signal is used to unload generation. The inertia of rotating equipment is used to buffer the system with rotational frequency rising above 60 cycles when supply exceeds demand and frequency declining below 60 cycles when demand exceeds supply—thereby achieving a new energy balance between supply and load at the new frequency. An uncontrolled unbalance in grid supply and demand (an unstable condition) would cause widespread physical damage when rotating equipment is forced to operate outside the rotational speed design parameters (typically 57-62 cycles). These controls are not readily applicable to a fuel cell stand alone power supply.